1. Field of the Invention
The present invention relates to a stepping motor, and more specifically to a PM (permanent magnet) type stepping motor.
2. Description of the Related Art
FIGS. 12 and 13 are respectively partly cutaway perspective and cross sectional views of a conventional PM stepping motor 30.
The PM stepping motor (hereinafter referred to as “stepping motor” as appropriate) 30 includes a stator assembly composed of first and second stators fixedly attached to each other. The first stator includes a bobbin 33a, a coil 34a wound around the bobbin 33a, and first (outer) and second (inner) yokes 31a and 32a disposed to house the bobbin 33a and the coil 34a, and the second stator includes a bobbin 33b, a coil 34b, and first and second yokes 31b and 32b (outer and inner yokes) disposed to house the bobbin 33b and coil 34b. 
The stepping motor 30 also includes a rotor assembly composed of a ring magnet (permanent magnet) 38 multipole-magnetized at its circumference, a rotor sleeve 37 fixedly disposed inside the ring magnet 38, and a rotary shaft 39 fixedly inserted through the rotor sleeve 37.
A front plate 20 is fixedly attached to the axially outer side (the first yoke 31a) of the first stator, and a front bearing 35a to rotatably support the rotary shaft 39 of the rotor assembly is fixedly attached to the front plate 20, while a rear plate 21 is fixedly attached to the axially outer side (the first yoke 31b) of the second stator, and a rear bearing 35b to rotatably support the rotary shaft 39 is fixedly attached to the rear plate 21. Thus, the rotary shaft 39 of the rotor assembly is rotatably supported by the front and rear bearings 35a and 35b, whereby the magnet 38 and the rotor sleeve 37 mounted on the rotary shaft 39 are adapted to rotate in a cylindrical hollow space formed in the stator assembly. The stepping motor 30 further includes front and rear spacers 36a and 36b having slidability.
A plurality of pole teeth are formed extending from the first yoke 31a/31b so as to intermesh with a plurality of pole teeth formed extending from the second yoke 32a/32b. The pole teeth are located close to the outer circumference of the magnet 38. When a pulse voltage is applied to the coils 34a and 34b, a magnetic field is produced at each pole tooth of the first and second yokes 31a and 32a and the first and second yokes 31b and 32b, and a driving force to rotate the rotary assembly is generated by the magnetic field produced at the pole tooth and the magnetic field of the magnet 38.
Since the stepping motor 30 includes the front plate 20 with the front bearing 35a and the rear plate 21 with the rear bearing 35b as described above, the axial dimension of the body section of the stepping motor 30 is inevitably increased by the aggregate thickness of the front and rear plates 20 and 21. So, if the stepping motor 30 has an upper limit to the axial dimension of its body section, the ratio of the volume of the coils 34a and 34b relative to the axial dimension of the body section is decreased, resulting in restriction of its output torque. Also, since the front plate 20 functions as a motor attachment plate and has its configuration modified according to customers' specifications, different production procedures are required for the different configurations of the front plate 20, which makes it difficult to standardize the production procedure, and the motor structure as well.
To deal with the problem described above, a stepping motor is disclosed in which front and rear bearings are attached directly to the inner circumferences of first yokes without using front and rear plates (refer to, for example, Japanese Patent Application Laid-Open No. 2006-254557; page 3 and FIG. 1 therein).
FIG. 14 shows a stepping motor 40 which eliminates front and rear plates as described above. The stepping motor 40 includes: a rotor assembly composed of a rotor magnet (twin magnets) 41 and a rotary shaft 42; and a stator assembly made up of two stators 49 and 50 each of which is composed of an first yoke 47/48 made of a soft magnetic material and having pole teeth 47a/48a arrayed circumferentially, a second yoke 43/44 made of a soft magnetic material and having pole teeth 43a/44a arrayed circumferentially, and an exciting coil 45/46. And, the stepping motor 40 further includes sintered oil-impregnated bearings 51 and 52 fixedly attached respectively to the first yokes 47 and 48 and adapted to rotatably support the rotary shaft 42.
Referring to FIG. 14, the bearing 51 is made up of two portions, specifically a base portion 51a having a large diameter and a boss portion 51b having a small diameter. The bearing 52 has the same structure as the bearing 51 (though not revealed in the figure). The bearings 51 and 52 are fixedly attached respectively to the first yokes 47 and 48 such that the boss portions 51b and 52b are engagingly inserted into the circular hollow spaces defined by the inner circumferences of the first yokes 47 and 48, and such that the base portions 51a and 52a are brought into contact with the axially outer faces of the first yokes 47 and 48 thus properly fixing the axial position.
When an axial load is applied to the rotary shaft 42, or an external impact is accidentally given to the stepping motor 40, it can possibly happen that the bearings 51 and 52 come off the first yokes 47 and 48. Also, even if an adhesive is applied between the bearings 51 and 52 and the first yokes 47 and 48, the bearings 51 and 52 still can possibly come off the first yokes 47 and 48 due to the deterioration of the adhesive resulting from time and temperature changes.
To overcome the coming-off problem, the bearings 51 and 52 may be made of a sintered oil-impregnated iron material, engagingly inserted into the open spaces inside the first yokes 47 and 48, and securely fixed to the first yokes 47 and 48 by welding. However, the stepping motor 40 still has the following disadvantage.
The bearing 51/52, when duly attached to the first yoke 47/48, protrudes axially for the thickness of the base portion 51a/52a thus increasing the axial dimension of the stepping motor 40. The smaller the outer diameter of the stepping motor 40 is, the larger the ratio of the diameter of the base portion 51a/52a of the bearing 51/52 becomes with respect to the outer diameter of the stepping motor 40. Consequently, if the axial dimension of the stepping motor 40 is subject to restriction, the ratio of a volume occupied by the coil 45/46 relative to the axial dimension of the body section is small thus making it difficult to provide a sufficient motor torque as in the stepping motor 30 of FIGS. 12/13.